Typical conventional magnetic heads include those for use with mini-disks (hereinafter referred to as MDs). One example of such magnetic heads for MDs is disclosed by JP 7(1995)-129908A.
The MD is a kind of perpendicular thermomagnetic recording media employing magneto-optical recording techniques. In the magneto-optical recoding techniques, for recording, a medium locally heated by a laser beam to have a reduced coercive force is magnetized by applying thereto a perpendicular magnetic field modulated according to recording signals, so that perpendicular magnetic domains are formed therein. This modulated perpendicular magnetic field is generated by a magnetic head. For reproduction, the rotations of planes of polarization due to the Kerr effect of reflected light are detected so that magnetization directions of perpendicular magnetic domains are read.
MDs also are required to have higher transfer rates mainly in the case where they are used for data, pictures, etc., and this recently results in that modulated magnetic fields of higher frequencies are demanded.
The following will describe a configuration and operations of a conventional magnetic head for use in the magneto-optical recording technique, while referring to FIG. 8, which is a cross-sectional view of principal parts of the magnetic head. In FIG. 8, 1 denotes a recording medium such as a MD composed of a substrate 1a, a recording film 1b, and a protective film 1c. More specifically, other constituent elements such as a sliding film, a reflection film, etc. are included, but they are omitted herein. The recording medium 1 is moved by a mechanism (spindle motor, etc.), not shown in the drawing, in a direction indicated by an arrow A.
2 denotes an objective lens. The objective lens 2 allows a laser beam L from a light source to pass through the substrate 1a, and converges and focuses the same onto the recording film 1b. 
51 denotes a coil as a source of magnetomotive force. 52 denotes a magnetic core made of a soft magnetic material. As a soft magnetic material, a ternary-compound-oxide magnetic material such as MnZn ferrite or NiZn ferrite is used preferably, which has relatively excellent high-frequency characteristics.
The magnetic core 52 is formed in an approximate “E” shape in which one center yoke 52a and two side yokes 52b are connected with one another via a base yoke 52c, and a coil 51 is wound and fixed around the center yoke 52a. A magnetic flux generated by the passage of electric current through the coil 51 is guided by the magnetic core 52, thereby causing a perpendicular magnetic field with an intensity necessary for recording to be applied to the recording film 1b. The coil 51 and the magnetic core 52 compose a magnetic head.
With respect to a MD, the magnetic core 52 is kept out of contact with the protective film 1c so as to avoid damage due to collision with the protective film 1c. However, from the viewpoint of the power consumption, the coil 51 preferably is arranged as close to the protective film 1c as possible in an acceptable range so that the efficiency of converting the driving current of the coil 51 into the magnetic field applied to the recording film 1b is increased.
Furthermore, assume that a height and a width on one side of a cross section of the coil 51 shown in the drawing are h0 and w0, respectively. In order to increase the efficiency of the conversion from the current of the coil 51 to the magnetic field, in the case where h0×w0 is set constant, that is, a cross-sectional area occupied by the coil 51 is set constant, it is more effective that w0/h0 decreases, that is, that the cross section of the coil 51 in the drawing have a more-vertically-elongated shape, in a certain range. This is because the cross section of the coil 51 in a vertically-elongated shape means that a width w1 of a magnetic gap that is a space between the center yoke 52a and the side yoke 52b decreases, which allows a magnetic resistance of an entirety of the magnetic head to decrease, thereby improving the efficiency. In the case of the shape shown in FIG. 8, w0/h0 is approximately 0.5.
Furthermore, in the case where the coil 51 has a cross section in a vertically-elongated shape, a mean diameter of the coil 51 decreases, which allows the center of the cross section of the coil 51 to be arranged closer to the center yoke 52a, thereby improving the efficiency also. In this case, the decrease of the mean diameter of the coil causes the coil resistance to decrease also, thereby reducing the power consumption of the coil.
When data are recorded, the coil 51 is modulated by a current according to a recording signal so as to generate a magnetic flux. The recording film 1b is subjected to a modulated magnetic field by the magnetic flux guided thereto by the magnetic core 52. Here, the laser beam L is converged and focused by the objective lens 2 on the recording film 1b, thereby heating the recording film 1b, causing its coercive force to decrease. Therefore, the record therein before the heating is erased. When the recording medium 1 moves in the arrow A direction, the temperature of the recording film 1b drops. As a result, the coercive force is recovered, and a modulated magnetic field exerted thereto currently is recorded.
However, the above-described conventional magnetic head has the following problems. When the modulated magnetic field is applied to the magnetic core 52, losses are produced, and mainly are converted into heat, thereby causing the temperature of the magnetic core 52 to rise. These losses include an eddy current loss and a hysteresis loss, which are present independently from a so-called copper loss produced due to the resistance of the coil per se. Considering the hysteresis loss in particular, in the case where the material characteristics are assumed to be constant, the loss is considered to be proportional to an integral of a magnetic flux density with a volume.
Since an increase in a modulation frequency leads to an increase in the number of hysteresis loops within a unit time, the energy consumption within a unit time increases. This causes the temperature of the magnetic core to rise. Further, since an increase in the dimension of h0 leads to an increase in the volume in an area with a high magnetic-flux density, an energy-consuming area of the core increases.
Generally, the magnetic permeability of a magnetic material has a temperature-dependent characteristic; the magnetic permeability abruptly drops when the temperature exceeds a certain level, and approximates the absolute permeability of vacuum when the temperature is in the vicinity of a Curie temperature. In other words, as the temperature of the magnetic core rises, a magnetic resistance increases, and a magnetic field is not generated sufficiently. Furthermore, in the case of a low-cost coil employing an insulation coating with a low heat resistance, there is a possibility that the coil could be burnt out by heat generated by the magnetic core. Furthermore, an increase in the magnetic resistance causes the number of interlinkage fluxes to decrease, thereby reducing an inductance of the magnetic head. Accordingly, in the case of a low-cost constant-voltage circuit or the like, the current passing through the coil increases due to a decrease in an impedance of the magnetic head, thereby generating further more heat, and sometimes causing a so-called thermal runaway state, which burns out the coil, and breaks down the circuit, etc. Consequently, the above-described conventional magnetic head, which is designed only for reducing an absolute amount of current, has a drawback in that the magnetic head is not suitably employed for high-frequency magnetic-field modulation recording.